Your leak could be coming from anywhere, not necessarily your seam. Chances are it is coming from multiple locations. I have a 17 ft. Boler
that is presently gutted and I have found numerous places where water has slowly "wicked" in over the years. Most of the leaks
are located wherever screws were driven through the fibreglass shell and missed their backing plate. Chances are things are similar with the Bigfoots.
As ripping your entire interior out is probably not high on your list of options I'd suggest trying to access some common points where screws have been driven through. ie: around your running/signal lights
, around your ceiling vent, around your windows
etc., and then attempting to re-seal these points , from either the interior or exterior, using fibreglass resin and "chop" fibre cloth.
As to your seam, if it is similar to the seam on the Bolers, you could fill the gap between the two halves with Bondo Hair.
3M 762 Bondo-Hair Long Strand Fiberglass Reinforced Filler - Quart
Bondo® Polyester Fiberglass Resin (401) - Ace Hardware
Bondo(R) Fiberglass Mat, 487, 32 sq ft, 6 per case
Take a look at this video of the disassembly of my 17 ft. Boler
YouTube - dismantling Interior part 1.wmv
At 29 seconds in you can see the damage caused by water seeping in through the screw holes for years. As well, at 6:04 into the video you can see where a whole row of screws missed their "mark" allowing water to seep in.